1. Field of the Invention
The present invention relates to a novel azafluorene derivative and an organic light-emitting device using the derivative.
2. Description of the Related Art
An organic light-emitting device is a device having a thin film which contains a fluorescent or phosphorescent organic compound and is interposed between electrodes. Electrons and holes are injected from the respective electrodes, whereby excitons of the fluorescent or phosphorescent compound are produced. The excitons emit light when they return to their ground states.
The development of an organic light-emitting device is remarkable, and the characteristics of the device are such that luminance is high at low applied voltage, lights with various wavelengths can be emitted, the speed of response is high, its thickness is thin and its weight is light. Accordingly, the device has potential for a variety of applications.
However, the present situation calls for optical output with higher luminance or higher conversion efficiency. In addition, many problems still remain unsolved regarding durability against changes over time due to long-term use, and deterioration caused by atmospheric gas containing oxygen or moisture. Further, when considering application to a full color display, the present art is still insufficient to address problems concerning need for light emission of blue, green, and red with high color purity.
In the organic light-emitting device, an electron transport layer in contact with a light-emitting layer on the cathode side, an electron injection layer and a hole-blocking layer are not directly involved in the light emission of the device. However, each of the layers largely affects the light-emitting characteristic and durability of the device from the viewpoint of, for example, a carrier balance in the light-emitting layer. In general, heterocyclic derivatives are often used in the electron transport layer.
Azafluorene derivatives are included in the heterocyclic derivatives, and the case where an azafluorene derivative is used in an organic light-emitting device is described in, for example, each of Japanese Patent Application Laid-Open No. 2005-314663, WO 2006/081973, and Org. Lett., 8, 1415 (2006). In each of Japanese Patent Application Laid-Open No. 2005-314663 and WO 2006/081973, an azafluorene derivative is used as a ligand of a metal complex in an organic light-emitting device. Org. Lett., 8, 1415 (2006) proposes that an azafluorene derivative whose azafluorene ring is substituted by an aryl group at the 9-position is used in an organic light-emitting device. However, there is no report on an example in which an azafluorene derivative obtained by substituting 9-position of an azafluorene ring by an alkyl group is used by itself and not as a ligand of a metal complex in an organic light-emitting device, in particular, in an electron transport layer.